Hi5 has an API server !
In the next few days we're going to roll it out for widget integration. Rock You and Slide are both talking to it. We're interested in working with a few more select partners.
The one thing we're focusing on is official and de-facto standards. That includes building Atom feeds for everything, FOAF, OpenID, etc.
The entire thing is built on Enunciate, which is a seriously cool framework.
